The invention relates to a method of controlling the concentration level of a potentially harmful metal, such as lead, at which a positive response is obtained when using a chemical test kit for the detection of that potentially harmful metal in paint or other media. A positive response means that the potentially harmful metal is determined to be present in the material being tested. The method is independent of the chemistry on which the test kit is based.
The discussion of this invention focuses on the use of chemical test kits for detection of lead in paint and other coatings. However, the invented method is also applicable to testing any potentially harmful metal using any chemical test kit that has been found to generate a positive response at concentration levels well below a level considered harmful, i.e., a test kit that is too sensitive with respect to a defined action level. The invented method can also be used for detection of potentially harmful metals in any solid medium other than paint or other coatings provided that the medium is not so hard as to damage the grinder which is one of the tools used by the invented method.
According to a report by the President's Task Force on Environmental Health Risks and Safety Risks to Children, approximately 24 million U.S. dwellings were at risk for lead-based paint hazards in 1999 [“Eliminating Childhood Lead Poisoning: A Federal Strategy Targeting Lead Paint Hazards,” President's Task Force on Environmental Health Risks and Safety Risks to Children (February 2000)]. The term lead-based paint (LBP) means paint or other surface coatings that contain lead equal to or exceeding a level of 1.0 milligram per centimeter squared or 0.5 percent by weight [Public Law 102-550, Residential Lead-Based Paint Hazard Reduction Act of the Housing and Community Development Act of 1992, Available from Superintendent of 63 Documents, U.S. Government Printing Office, P.O. Box 371954, Pittsburgh, PA 15250-7954; www.access.gpo.gov/su-docs.]. This definition is also given in the Guidelines for the Evaluation and Control of Lead-Based Paint Hazards in Housing, a document often called the HUD Guidelines [HUD- 1539-LBP, U.S. Department of Housing and Urban Development, Washington, DC (July 1995)]. The accurate and efficient identification of LBP in housing is important to the Federal government. For example, identification of LBP in most pre-1978 target housing requires disclosure of that information, if available, to the owner, prospective purchasers, or tenants (42 U.S.C. 4852d, 24 CFR 35.80-98). Also, in certain target housing receiving financial assistance from HUD, or being sold by the Federal government, identification of LBP results in requirements for LBP hazard evaluation and/or control (42 U.S.C. 4822, 24 CFR 35.1-1355). As far back as the early 1970s, chemical test kits were introduced as relatively nonintrusive, potentially cost saving, qualitative methods for determining the presence or absence of LBP while on-site [Vind, H. P. and Drisko, R. W., Field Identification of Weathered Paints, Technical Report TR-766, Naval Civil Engineering Laboratory, Port Hueneme, Calif. (April 1972), 21 pages; Vind, Harold P. and Mathews, C. W., Field Test for Detecting Lead-Based Paint, Technical Note N-1455, Civil Engineering Laboratory, Port Hueneme, Calif. (September 1976), 9 pages].
A (chemical) test using a test kit involves the application of chemical solution to a prepared dry paint film sample, paint chip, paint powder, or painted surface and the subsequent observation of the presence or absence of a characteristic color change [Use of Qualitative Chemical Spot Test Kits for Detection of Lead in Dry Paint Films, Annual Book of ASTM Standards, Standard Practice E 1753, Vol. 04.07, American Society for Testing and Materials, West Conshohocken, Pa. (1998)]. The most commonly used types of test kits for detecting lead in paint and other coatings involve either rhodizonate or sulfide ion. Several U.S. patents and patent applications exist for such test kits [U.S. Pat. Nos. 6,800,485, 6,489,170, 5,558,835, 5,550,061, 5,364,792, 5,330,917, 5,278,075, 5,039,618, U.S. patent application Ser. No. 2011/0283785, U.S. patent application Ser. No. 2003/0203496, and U.S. patent application Ser. No. 2003/0049852] and prepackaged kits covering both of these types are commercially available from a number of suppliers. The first type is based on the reaction of rhodizonate ion with lead II ion; in acidic solution this reaction produces a color change from yellow-orange to pink or red. The other is based on the reaction of sulfide ion, in basic solution with lead II ion; where either solid lead (II) sulfide is created producing a color change to gray or black or a dark colored solution is formed by adding a caustic leach of the sample prior to adding the sulfide ion [U.S. patent application Ser. No. 2011/0283785]. Observation of the characteristic color change is a taken as a positive indicator of the presence of lead in the paint sample tested.
The American Society for Testing and Materials (ASTM) has issued two standards associated with the use of test kits for LBP: ASTM E 1753, Practice for Use of Qualitative Chemical Spot Test Kits for Detection of Lead in Dry Paint Films [Standard Practice E 1753, Vol. 04.07, American Society for Testing and Materials, West Conshohocken, Pa. (1998).] and ASTM E 1828, Guide for Evaluating the Performance Characteristics of Qualitative Chemical Spot Test Kits for Lead in Paint [ASTM Standard E 1828, Annual Book of Standards, Vol. 04.07, American Society for Testing and Materials, West Conshohocken, Pa. (1998)].
Potential advantages to using test kits over other methods of identifying LBP are that they: are inexpensive and rapid; may require minimal operator technique; and may respond to microgram levels of analyte [Luk, K. K., Hodson, L. L., O'Rouke, J. A., Smith, D. S., and Gutknecht, W. F., Investigation of Test Kits for Detection of Lead in Paint, Dust, and Soil, EPA 600/R-93/085, U.S. Environmental Protection Agency, Research Triangle Park, NC (April 1993)]. For a test kit to be suitable for rapid in-field (not in a laboratory) determinations, the steps needed to conduct the test must be practical, simple, inexpensive, and suitable for use by persons with little or no specific training in laboratory procedures.
Various studies have shown that existing test kits are not reliable for identifying LBP [A Field Test of Lead-Based Paint Testing Technologies: Technical Report, EPA 747-R-95-002b (May 1995); Estes, E. D. and Gutknecht, W. F., Workshop Report: Identification of Performance Parameters for Test Kit Measurement of Lead in Paint, EPA 600/R-93/129, U.S. Environmental Protection Agency, Research Triangle Park, NC (June 1993); Gutknecht, W. F., Hodson, L. L., Luk, K. K., Binstock, D. A., Van Hise, C. C., and Turner, A. R., Pilot Field Study for the Assessment of Techniques Used for the Measurement of Lead in Paint, EPA 600/R-97/057, U.S. Environmental Protection Agency, Research Triangle Park, NC (December 1997).]. The main issue is that, while several kits have been shown to reliably produce positive responses at lead levels at or above the Federal regulatory definition of LBP, all of them also generate positive response well below that level. In other words, existing chemical test kits tend to be too sensitive and often generate positive results even when the true level of lead in the paint is well below the level defined as LBP in Federal regulations.
On Apr. 22, 2008, EPA issued a rule (the Renovation, Repair, and Painting Program Final Rule [the RRP Rule]; Federal Register: Apr. 22, 2008, Volume 73, Number 78, pages 21691-21769). The RRP Rule requires the use of lead-safe practices and other actions aimed at preventing lead poisoning. The RRP Rule allows certified renovators to use EPA recognized chemical test kits as a method of determining the presence or absence of LBP for renovators. Specifically, the final rule exempts renovations (from using lead-safe work practices) that affect only components that a certified renovator, using a chemical test kit recognized by EPA, determines are free of LBP. Since there is a cost to using lead safe work practices, an inexpensive method that can successfully determine the presence or absence of LBP is desirable to reduce the cost of renovation. Laboratory analysis of submitted paint samples could be used, but such an approach is considered far too expensive, too time consuming, and requires a certified lead professional to collect the samples. In response to the need, EPA set a goal within the RRP rule to foster the development of a chemical test kit that can reliably be used by a person with minimal training, is inexpensive, provides results within an hour, and is demonstrated to have a false positive rate of no more than 10 percent and a false negative of less than 5 percent at 1.0 milligram/centimeters squared or 0.5 percent by weight. The requirement for both low false positive and low false negative rates is sometimes referred to as a two-sided test (all quantitative laboratory measurements for metals in paints and coatings provide two-sided tests). EPA further stated in the rule that they were confident that improved test kits meeting EPA's benchmarks would be commercially available by September 2010. EPA's Environmental Technology Verification (ETV) Program was set up as the federal vehicle to evaluate the performance of vendor-submitted lead test kits. Currently, no submitted test kit is able to meet the EPA requirements for a 2-sided test. In the interim, a test kit can be EPA-recognized if it meets the negative response criterion of no more than 5 percent false negatives, with 95 percent confidence for paint containing lead at or above the regulated level, 1.0 mg/ milligram/centimeters squared or 0.5 percent by weight. In other words, the interim recognition is being offered by EPA to test kits that can be used as a one-sided test, referred to as a negative screen, where a negative response it highly likely to indicate that no lead is present at or above the definition for LBP. A positive result from such a test kit is interpreted to mean that the presence of LBP is undetermined (and lead safe work practices would be required). The interim recognition will last until EPA publicizes recognition of the first test kit that meets both the negative response and positive response criteria. As of June 2012, three test kits have been given interim recognition (see <http://www.epa.gov/lead/pubs/testkit.htm>; last accessed Jun. 15, 2012). The fact that there are test kits able to achieve only interim recognition is proof that these chemical test kits tend to be too sensitive and give positive responses at levels well below the Federal definition of LBP.
The invention, use of solid-phase sample dilution to prepare a sample prior to testing using a test kit, addresses the two key problems with current chemical test kits for the determination of the presence or absence of LBP. The first problem is conversion of the paint sample into particles small enough for the acid or base, present in the chemical test kits, to effectively dissolve the lead in the sample and convert all lead present into lead (II) ions. The chemical reactions used in test kits to detect the metal require that the lead in the sample be dissolved in solution so that all the lead is present in ionic form so it can react with test kit chemicals to create a colored complex that is used to signify presence of the metal in the sample undergoing testing. Breaking down a paint sample into small particles is difficult partly because real-world paint samples from housing and other dwellings often have over-layers of acrylic (latex) paint and these layers tend to make the paint gummy or sticky, rendering it hard to break up. In a laboratory setting, there are many tools available to a trained laboratory technician to convert a solid paint sample into small particles, such as freezing the sample and breaking it up in a shatter-box or using a mortar and pestle. In addition, strong acids can be safely used in a laboratory vented hood to digest the sample and put the lead into solution and these strong acids can help compensate for inefficient breakup of the sample. However, in a non-laboratory setting (such as a home) these tools are impractical, potentially hazardous and generally cannot be successfully or safely used by someone other than a qualified laboratory technician.
The second common problem that existing chemical test kits have is controlling the amount of (paint) sample that is exposed to the chemicals in the test kit. For most test kits, the human eye is used as a detector to produce one of two responses to lead present in the sample undergoing testing: a negative, a color change is not observed; and, a positive, a color change is observed. Positive detection of lead occurs when there is sufficient lead (II) ions present to form enough colored complexes with the chemicals present in the test kit to be observable by the eye. In addition to the color change itself, the degree of color change is sometimes used to help make more accurate negative and positive assessments, generally taking the form of using a colored or shaded standard to use for comparison in a manner that is similar to using pH paper to determine the pH of a solution. However, adjusting the lead level in the paint at which a color change can be observed, by means of a change in the test kit chemicals, is difficult. Since most test kits tend to be too sensitive, the only practical method of obtaining an observable color change at a higher lead level is to reduce the number of lead (II) ions exposed to the chemicals through a reduction in the amount of sample that is undergoing testing. In a laboratory setting, this is generally done by diluting a dissolved (liquid) paint sample until the number of lead (II) ions present in the tested sample is within the concentration operating range of the analytical technique used to measure the amount of lead in the diluted (liquid) sample. Another common laboratory method is to dissolve a sub-sample of the paint sample (by mass) after it has been broken down into smaller particles (discussed under the first problem above). Use of liquid serial dilution methods in the field is impractical, costly, and likely well beyond most persons who are not qualified laboratory technicians. Likewise, the sub-sampling approach is also impractical as it requires the collection of whole sample and sub-sample masses using a high quality analytical scale, again requiring expensive equipment and laboratory level training Current and past test kits have attempted to reduce the number of lead (II) ions exposed to the chemicals by reducing the amount of paint being measured through the use of coring tools with specific diameters and various paint sample slicing methods where specific widths and lengths of paint are exposed to the test kit chemicals. Both these approaches suffer from an inherent problem with using a smaller collected paint sample, i.e., if the collected sample is too small, it may not be representative of the surface material that is being evaluated for lead content. For example, the D-Lead kit presented in U.S. patent application Ser. No. 2011/0283785, uses a 3/16 inch diameter coring tool to obtain a small sample of paint to be exposed to the chemical reagents used in the kit. Given the variable thickness often found in paint and coatings, caused by typical painting techniques, this small diameter (representing an area of only 0.18 centimeters squared) can easily fail to collect a representative sample. In addition, as evidenced by the fact that this kit was unable to meet all the EPA RRP Rule requires, this paint sample size is still too large to reliably determine the presence of LBP (see the estimated false positive rates of 16-29 percent quoted in the application). Using the invention presented here, it is estimated that the area of the sample for the D-Lead kit would have to be reduced by more than a factor if 4 to have a chance at meeting the EPA's less than 10 percent false positive rate criterion. Such a small sample (less than 0.1 inches in diameter) would be hopelessly unrepresentative, and collecting it with the needed repetitive accuracy inherently impractical. As of June 2012, none of the approaches used by test kit manufacturers have been successful as proven by the inability of any of the test kits to achieve two-sided performance requirements set forth by EPA in the RRP Rule.